The small and transient metabolic increase associated with acute nicotine intake has been well documented, but only one study has investigated the cumulative effect of repeated nicotine intake characteristic of smoking. This pilot study of eight smokers (4 men, 4 women) focuses on 24-hour changes in energy expenditure, substrate oxidation and related biochemical processes after one week of complete nicotine abstinence. The protocol involves two 24-hour stays in the GCRC metabolic chamber under carefully controlled diet and exercise conditions, with subjects using either the double strength 4mg nicotine gum or a placebo gum (for 30 minutes every hour) and repeated baseline blood testing after 9 hours of nicotine gum use. This will be the first attempt at understanding postcessation weight gain by investigating how one week of nicotine deprivation affects 24-hour energy expenditure as well as substrate oxidation processes (e.g., fat metabolism), as compared to cumulative day-long nicotine use.